The present invention relates to methods for hydrolyzing water-soluble polymers that normally hydrolyze when contacted with base.
The use of partially hydrolyzed polyacrylamides and other carboxamide polymers (often called anionic polyacrylamides) in such applications as enhanced recovery of petroleum, flocculation of finely divided solids from aqueous suspension and the like is well known. In such applications it is often desirable to convert from about 1 to about 65 mole percent of the carboxamide groups of the polymer to carboxy groups.
For the production of high molecular weight, partially hydrolyzed polyacrylamides and other carboxamide polymers, it has been suggested that the carboxamide polymer in aqueous solution be hydrolyzed to the desired extent by contacting the aqueous solution with alkali such as sodium hydroxide. However, the preparation of a homogeneous hydrolyzed carboxamide product by such method is difficult because of the high viscosities of aqueous solutions of such polymers. Such high viscosities hinder and often prevent uniform mixing of the alkali with the aqueous solution of the polymer. Thus in such a method of operation, it is generally found that some of the polymer is hydrolyzed to a greater extent than desired while other portions of the polymer undergo little or no hydrolysis.
In order to overcome such problems of non-uniform hydrolysis, partially hydrolyzed carboxamide polymers have been prepared by polymerizing the carboxamide monomer in an aqueous medium containing sodium hydroxide or sodium carbonate (see U.S. Pat. No. 2,820,777 and 3,022,279). While these methods do improve the uniformity of hydrolysis, the presence of the sodium hydroxide or the sodium carbonate may interfere to some extent with the polymerization thereby hindering if not preventing the formation of the very high molecular weight polymers that are desired.
Other attempts to overcome the problems of non-uniform hydrolysis have involved the formation of an emulsion of the aqueous solution of the polymer in oil (so-called water-in-oil emulsions) and reacting that emulsion with aqueous solutions of base such as sodium hydroxide. See German Patent Application No. 2,535,350. Unfortunately this procedure does not totally overcome the problems of non-uniform hydrolysis over wide ranges of reaction conditions that are often desirable to employ in the hydrolysis of carboxamide polymers. In particular it is generally necessary to carry out the process using rather dilute (e.g., less 35 weight percent) aqueous solutions of the polymer in order to avoid significant destabilization of the water-in-oil emulsion.
In view of the aforementioned deficiencies of the prior art methods, it would be highly desirable to provide an improved method for hydrolyzing carboxamide polymers under conditions of rapid hydrolysis whereby the resulting polymer product exhibits the desired stability, uniform degree of hydrolysis and high molecular weight.